1. Technical Field of the Invention
This invention relates to an exhaust gas temperature control system and, more particularly, to a method and apparatus for improved exhaust gas temperature control within an exhaust system of a two-stroke engine in order to improve engine performance.
2. Description of the Prior Art
Small two-stroke engines, or two-cycle engines, as they are also sometimes referred to, are commonly found in motorcycles, personal watercraft, such as JET SKIs, snowmobiles, and the like. Two-stroke engines are desirable for these types of vehicles in that they provide high output, have a low weight contained within small packaging, and are economical to produce.
Basic operation of a two-stroke engine requires two piston strokes or only one revolution for each cycle. Exhaust ports in the cylinder wall are uncovered by the piston, or exhaust valves in the cylinder head are open near the end (at 60%-88%) of the expansion stroke, permitting the escape of exhaust gases and reducing the pressure in the cylinder. A charge of air or combustible mixture flows into and is compressed in a separate crankcase compartment for each cylinder to slightly above atmospheric pressure. Intake ports are uncovered by the piston or intake valves and are opened soon after the opening of the exhaust, and the compressed charge flows into the cylinder, expelling most of the exhaust products, some unburned charge escaping with the exhaust. Just before the piston reaches top dead center, the unburned charge within the cylinder ignites, which in turn drives the piston downward. As the piston moves down, the exhaust port in the cylinder wall is exposed. The hot exhaust gas is released into the exhaust system via this exhaust port. However, also released at this time is a high pressure pulse. This pressure pulse is a sonic wave that travels at the speed of sound independently of the exhaust gas itself. The exhaust system has an expanding and diverging shape to its initial section, and the sonic wave travels the length of this initial section until it reaches a second section of the exhaust system which has a converging shape. Once the sonic wave reaches this converging section, it is reflected back toward the exhaust port. During this time the piston has traveled downwardly, thereby exposing the transfer ports that allow incoming fresh charge to replace the outgoing burned charge. A well-tuned exhaust system will draw out all of the burned charge and, as previously mentioned, a bit of the fresh unburned charge into the exhaust system. Such a well-tuned exhaust system will utilize the aforementioned pressure pulse that is being returned by the converging section to push this fresh charge back into the cylinder. This results in higher-power output, lower emissions, and better fuel economy. As can be readily discerned, timing is critical since if the pressure pulse is returned too late or too early, the benefit of the tuned exhaust system is at best diminished, and at worst the exhaust system is a detriment. Given that the engine rpm varies, it is easy to see the value in varying the time required for the pressure wave to return. This may be accomplished by changing the distance the wave must travel or by changing the speed at which it travels. Electronically controlled water injection (ECWI) deals with changing the speed at which the pressure pulse travels.
ECWI is a system developed by applicant that monitors engine rpm and injects water into the exhaust system in order to control exhaust gas temperature (EGT), which in turn affects sonic wave speed. Briefly, ECWI opens a solenoid valve at low engine speed that injects fluid, such as water, into the exhaust system, causing EGT to fall. Conversely, as engine rpms rise, the solenoid valve closes to thereby allow EGT to rise. The result of this process is an increase in low rpm power without affecting top speed, while the downside is that such an "on/off" approach leaves a large rpm range where the slow sonic wave speed is too slow and the fast sonic wave speed is too fast. Ideally, a "gradual" approach to controlling EGT is desired.
Several ways of accomplishing such a gradual approach to controlling EGT have been attempted. Examples include electronic servo valves, mechanical valves with shifting spools, multiple solenoid valves in different locations or with varying spray nozzle sizes, and dedicated variable speed pumps. One of the better approaches involves utilizing a single solenoid valve and varying the amount of water it sprays into the exhaust system with pulsed width modulation, i.e. opening and closing the valve at high speeds (a frequency of about 50 Hz) and varying the percentage of "on" versus "off" time. This allows one to discretely vary water injected into the exhaust system with an inexpensive solenoid valve and to create a water injection modulation map that will provide the right amount of water for each rpm to thereby create a smooth power curve without the low power points on the curve. However, a major drawback with pulse width modulation lies in the fact that it tends to overcompensate in that the EGT (hence, the sonic wave speed) changes so quickly that in the time span of one pulse cycle, the EGT will go from too high (fast) to too low (slow) and back again. This compromises performance.
Finally, ECWI requires solenoid valves and spray nozzles with small orifices in order to operate correctly. This, of course, necessitates the use of a filter or filters. In the case of personal watercraft, filters are of particular importance since water directly from the lake is used for engine and exhaust system cooling. However, the engine cooling water requires no filtering since there are no small orifices to plug with debris from the lake. Thus, only the water supplied to the solenoid valves and spray nozzles needs to be filtered. Such filters, however, also need to be kept clean so that the supply of water to the ECWI system is not disrupted.
Accordingly, apparatus and a process are needed that control the exhaust gas temperature, and thereby the sonic wave speed, in a smooth and consistent manner.